Allergies are caused when your immune system reactions to things that are typically harmless to most people in an abnormal way, thinking that the harmelss substance is harmful to your body. Substances that cause allergic reactions are known as allergens. The immune system produces IgE antibodies in an attempt to protect the body. Those antibodies then cause certain cells in the body to release chemicals into the bloodstream, one of which is histamine.

The histamine then acts on a person's eyes, nose, throat, lungs, skin, or gastrointestinal tract and causes the symptoms of the allergic reaction. Future exposure to that same allergen will trigger this antibody response again. This means that every time you come into contact with that allergen, you'll have an allergic reaction.

The tendency to develop allergies is often hereditary, which means it can be passed down through your genes. You don't usually doesn't inherit a particular allergy, just the likelihood of having allergies.

Signs & Symptoms:

sneezing, often accompanied by a runny or clogged nose

coughing

itching eyes, nose and throat

dark circles under the eyes caused by increased blood flow near the sinuses

watery eyes

conjunctivitis (an inflammation of the membrane that lines the eyelids, causing red-rimmed, swollen eyes and crusting of the eyelids)

red, itchy skin

Some of the most common allergens are:

Foods. Food allergies are most common in infants and often go away as a child gets older. Although some food allergies can be serious, many simply cause annoying symptoms like an itchy rash, a stuffy nose, and diarrhea. The foods that people are most commonly allergic to are milk and other dairy products, eggs, wheat, soy, peanuts and tree nuts, and seafood

Insect bites and stings. The venom (poison) in insect bites and stings causes allergic reactions in many people. These allergies can be severe and may cause an anaphylactic reaction in some people.

Environmental Allergens. These are the most common allergens. Some examples of airborne particles that can cause allergies in people are dust mites; mold spores; animal skin; and pollen from grass and trees.

Medicines. Antibiotics — medications used to treat infections — are the most common types of medicines that cause allergic reactions. Many other medicines, including over-the-counter medications, can also cause allergic reactions.

Chemicals. Some cosmetics or laundry detergents can cause people to break out in an itchy rash (hives). Usually, this is because the person has a reaction to the chemicals in these products. Dyes, household cleaners, and pesticides used on lawns or plants can also cause allergic reactions in some people

Treatment:

The best treatment is avoidance:

Wear a pollen mask when mowing the grass or housecleaning

Stay indoors in the morning (when the pollen count is at its highest) and on windy days

Antihistamines: drugs that block the action of histamine, which is responsible for allergic symptoms. Short-acting antihistamines, which are generally over-the-counter (non-prescription), often relieve mild to moderate symptoms, but can cause drowsiness

Immunotherapy - When avoidance, environmental control measures and medications fail to control allergy symptoms, the doctor may suggest allergy immunotherapy ("allergy shots"). Immunotherapy involves the injections of allergen extracts to "desensitize" the person.